A CNGB1 Frameshift Mutation in Papillon and Phale`ne Dogs

A CNGB1 Frameshift Mutation in Papillon and Phalène
Dogs with Progressive Retinal Atrophy
Saija J. Ahonen1,2, Meharji Arumilli1,2, Hannes Lohi1,2*
1 Department of Veterinary Biosciences and Research Programs Unit, Molecular Neurology, University of Helsinki, Helsinki, Finland, 2 The Folkhälsan Institute of Genetics,
Helsinki, Finland
Abstract
Progressive retinal degenerations are the most common causes of complete blindness both in human and in dogs. Canine
progressive retinal atrophy (PRA) or degeneration resembles human retinitis pigmentosa (RP) and is characterized by a
progressive loss of rod photoreceptor cells followed by a loss of cone function. The primary clinical signs are detected as
vision impairment in a dim light. Although several genes have been associated with PRAs, there are still PRAs of unknown
genetic cause in many breeds, including Papillons and Phalènes. We have performed a genome wide association and
linkage studies in cohort of 6 affected Papillons and Phalènes and 14 healthy control dogs to map a novel PRA locus on
canine chromosome 2, with a 1.9 Mb shared homozygous region in the affected dogs. Parallel exome sequencing of a trio
identified an indel mutation, including a 1-bp deletion, followed by a 6-bp insertion in the CNGB1 gene. This mutation
causes a frameshift and premature stop codon leading to probable nonsense mediated decay (NMD) of the CNGB1 mRNA.
The mutation segregated with the disease and was confirmed in a larger cohort of 145 Papillons and Phalènes
(PFisher = 1.461028) with a carrier frequency of 17.2 %. This breed specific mutation was not present in 334 healthy dogs
from 10 other breeds or 121 PRA affected dogs from 44 other breeds. CNGB1 is important for the photoreceptor cell
function its defects have been previously associated with retinal degeneration in both human and mouse. Our study
indicates that a frameshift mutation in CNGB1 is a cause of PRA in Papillons and Phalènes and establishes the breed as a
large functional animal model for further characterization of retinal CNGB1 biology and possible retinal gene therapy trials.
This study enables also the development of a genetic test for breeding purposes.
Citation: Ahonen SJ, Arumilli M, Lohi H (2013) A CNGB1 Frameshift Mutation in Papillon and Phalène Dogs with Progressive Retinal Atrophy. PLoS ONE 8(8):
e72122. doi:10.1371/journal.pone.0072122
Editor: Ralf Krahe, University of Texas MD Anderson Cancer Center, United States of America
Received May 16, 2013; Accepted July 10, 2013; Published August 26, 2013
Copyright: ß 2013 Ahonen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was supported partly by the Academy of Finland, the Sigrid Juselius Foundation, Biocentrum Helsinki, the Jane and Aatos Erkko Foundation.
SA is a student in Helsinki Graduate Program in Biotechnology and Molecular Biology. The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: [email protected]
pigment epithelium-specific protein (RPE65) [12], solute carrier family 4,
anion exchanger, member 3 (SLC4A3) [13], progressive rod-cone degeneration
(PRCD) gene [14]. Furthermore, three X-linked canine PRAs are
known [15], [16]. However, there are still other PRA affected
breeds of unknown genetic cause.
Papillon breed is affected with an autosomal recessive late onset
PRA with a mean onset at 5.6 years of age [17]. Based on the
electroretinogram (ERG) recordings, affected dogs have a primary
loss of the rod photoreceptor cells, followed by loss of cone cell
function [18], [19]. The first clinical signs are seen as difficulties in
the dim light. The disease progress very slowly and the affected
dogs seem to be visually normal throughout their life, as the cone
function is fairly well preserved [18], [19]. The ophthalmoscopical
signs include increased tapetal reflectivity and retinal vascular
attenuation followed by pigment migration in the non-tapetal
fundus [17].
According to Fèdèration Cynologique Internationale (FCI),
Papillon breed is separated to two different breeds, Papillon and
Phalène in Europe. Separation is based on the ear morphology,
Papillons having bricked ears and Phalènes more low set ears.
Genetic background is expected to be similar as both types of dogs
are born in the same litter and puppies are registered in Europe
based on their ear morphology. The American Kennel Club
Introduction
Progressive retinal degeneration or atrophy is one of the most
common causes of blindness both in human and in dog affecting
the retinal photoreceptor cells. The most typical type of canine
progressive retinal atrophy (PRA) is primary rod degeneration,
where the light sensitive rod photoreceptor cells degenerate,
resulting visual impairment in the dark light. When the disease
progresses to the cone cells, which function in the day light, the
vision is completely lost [1]. Ophthalmoscopically the initial
clinical signs include tapetal hyperreflectivity, retinal vascular
attenuation and pigment changes and atrophy of the optic nerve.
[2]
Various forms of PRA have been diagnosed in different breeds
of dog, some breeds have been found to manifest more than one
type of the disease. Most canine PRAs are recessively inherited
and 12 causative genes are known including ADAM metallopeptidase
domain 9 (ADAM9) [3], bestrophin (BEST1) [4], chromosome 2 open
reading frame 71 (C2orf71) [5], cyclic nucleotide gated channel beta 3
(CNGB3) [6], nephronophthisis (NPHP4) [7], phosphodiesterase 6A,
cGMP-specific, rod, alpha (PDE6A) [8], rod cGMP-specific 3’,5’-cyclic
phosphodiesterase subunit beta (PDE6B) [9], rhodopsin (RHO) [10], retinitis
pigmentosa GTPase regulator-interacting protein (RPGRIP1) [11], retinal
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A CNGB1 Mutation in Dogs with Retinal Degeneration
by adjusting for the strongest SNP on CFA2 (BICF2P309315) to
detect the presence of any modifier loci.
Besides association, the genotyping data was analyzed using a
joint family-based linkage and association analysis program
Pseudomarker [22]. The family-based analyses were performed
under a recessive inheritance model, and included parametric
single-point linkage test, association analysis (LD|Linkage) and
joint analysis (LD+Linkage).
registers both types as Papillons. Due to shared breed origin, it is
reasonable to hypothesize that the genetic cause of PRA is shared.
We have established a pedigree and a study cohort of PRA
affected and unaffected Papillons and Phalènes to identify the
cause of the disease. We performed a genome wide association,
linkage and exome sequencing studies to map the disease to a
region in CFA2 and to identify a frameshift-causing indel mutation
in the cyclic nucleotide gated channel beta 1 (CNGB1) gene in the affected
dogs.
Exome sequencing
Materials and Methods
Exome sequencing was performed for a trio of Phalènes,
including an affected proband and healthy parent dogs (Fig. 1).
The coding sequences were captured using Agilent’s Canine
Exome Capture Kit (Agilent, Santa Clara, CA, USA) that was
designed based on the build 2.1 of the canine genome reference
sequence. The capture included a 54 Mb design that covered the
Ensemble and RefSeq Genes from the UCSC track, human
protein alignments and spliced ESTs that lie outside of Ensemble.
The capture was performed according the manufacturer’s
instruction and the sequencing was performed using the Illumina
HiSeq2000. Both the capture and sequencing were performed at
the FIMM technology center. The exome sequencing data is
available upon request.
The data analysis included quality control, alignment, variant
calling and annotation of the variants. Quality control was
performed using FASTX toolkit (http://hannonlab.cshl.edu/
fastx_toolkit/index.html) to remove the low quality raw-data and
to reduce the false positives, followed by alignment of the reads to
the build 2.1 of the canine genome reference sequence with
Burrows-Wheeler (BWA) aligner tool [23]. The aligned reads were
then directed to the variant calling programs GATK [24] and
Samtools [25] to identify the SNPs and short indels present within
the exome samples. Pindel [26] program was used to identify the
structural variants. Finally, the identified variants were annotated
against NCBI and Ensembl databases to find out the variants
present in the coding and non-coding regions and to the
dbSNP131 to identify known polymorphic variants. The annotation was performed using ANNOVAR [27] and in-house custom
R-scripts. The data was then filtered assuming a recessive mode of
inheritance using in-house R-scripts.
Study cohort
Blood and buccal swab samples were collected from pet dogs to
the Dog DNA bank at the University of Helsinki, Finland. All
samples were submitted by the owners’ consent and were collected
under the permission of animal ethical committee of County
Administrative Board of Southern Finland (ESAVI/6054/
04.10.03/2012). Genomic DNA was extracted from EDTA blood
samples, using Chemagic Magnetic Separation Module I (MSM I)
(Chemagen Biopolymer-Technologie AG, Baeswieler, Germany)
according to the manufacturer’s instructions. DNA from buccal
swabs (Eurotubo Cytobrush, sterile, 200mm, Danlab, Helsinki,
Finland) was extracted using QIAamp DNA Mini Kit (Qiagen). A
retinal sample from an Australian Cattle Dog, euthanized for
unrelated causes and free of PRA, was taken post mortem with
owner’s consent. RNA was extracted using RNeasy Mini Kit
(Qiagen) according to manufacturer’s instructions. DNA and RNA
concentrations were measured using Nanodrop ND-1000 UV/Vis
Spectrophotometer (Nanodrop technologies, Wilmington, Delaware, USA) and stored at -20uC (DNA) or 280uC (RNA).
We have collected altogether samples from 59 Papillons and
from 116 Phalènes in our Dog DNA bank. We selected clinically
confirmed cases for our genetic studies. Samples were collected
from affected Papillons (n = 4) or Phalènes (n = 2) with clinical
signs consistent with PRA, including tapetal hyper-reflectivity,
retinal pigmentary changes and vascular attenuation. Control dogs
(n = 14) in the mapping study were eye examined healthy over
seven years of age.
A pedigree was constructed around the affected dogs using the
genealogical data available in public canine registries such as the
Finnish Kennel Club’s Koiranet, the Swedish Kennel Club’s
Hunddata databases or as informed by the owners. A GenoPro
genealogy software was utilized to draw the pedigree.
Mutation screening and validation
An indel in exon 25, including 1-bp deletion followed by 6-bp
insertion was found in the cyclic nucleotide gated channel beta 1 (CNGB1)
and a non-synonymous variant in 2-oxoglutarate and iron-dependent
oxygenase domain containing 1 (OGFOD1) genes. These variants were
confirmed by PCR in additional cases and controls. PCR primer
pairs were designed using Primer3 [28] software to amplify the
variants from genomic DNA. The mutation on exon 25 of the
CNGB1 gene was amplified from 6 PRA affected and 14 control
Papillons and Phalènes and one Rough Collie using forward
primer 5’-AACAATCTCCTGGAGCCTCA-3’ and reverse primer 5’-TGGAAGCAACGGTAGAGAAGA-3’. The non-synonymous mutation on exon 9 of the OGFOD1 gene was amplified from
6 PRA affected and 10 controls dogs and one Rough Collie with
forward primer 5’-CTTGTGCTTGAGTTGGATGTG-3’ and
reverse primer 5’-TGGGAGAGCGAAGACTTTGT-3’.
The best associated SNP on CFA1 (BICF2P839236) was
amplified in a larger sample cohort of 6 affected and 168 controls
using forward primer 5’-TCCAATCCTTAATGTTCTAAAGTGAA-3’ and reverse primer 5’-CCCAGATTAAGATGATTCACCA-3’.
To further investigate the disease associated variant in a larger
sample cohort the CNGB1 variant was sequenced from 43
Genome wide association study
A genome-wide association study (GWAS) was performed using
Illumina’s CanineHD BeadChip arrays (San Diego, CA, USA) for
6 cases (4 Papillons, 2 Phalènes) and 14 controls (3 Papillons, 11
Phalènes). Genotyping was performed in our core facility at the
FIMM Technology Center. Quality control procedures were
included when analyzing the data. Only SNPs which met HardyWeinberg expectations P#0.0001, had $95% genotyping rate
and minor allele frequency (MAF) of 5% were included in the
analysis, resulting in the exclusion of 64640 SNPs out of 173662.
The GWAS data is available from the researchers upon request.
To compare the allele frequencies, a case-control association
test was performed using PLINK 1.07 [20]. Significance values
from these analyses were used to generate a whole-genome
association plot using the statistical package R [21]. Identity-bystate (IBS) clustering and CMH meta-analysis (PLINK) were used
to adjust for population stratification. Genome-wide corrected
empirical p-values were determined by applying 100.000 permutations to the data. The GWAS data was also re-analyzed (PLINK)
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Figure 1. Pedigree of PRA affected Papillons and Phalènes. Pedigree indicates the affected dogs that were used in the study. Samples from
six affected dogs were available for genotyping. Disease segregation is consistent with autosomal recessive mode of inheritance as all affected dogs
are born from healthy parents and both sexes are affected. Obligate carrier parents of affected dogs are marked in the pedigree. Obligate carriers
genotyped as heterozygous for CNGB1 mutation are marked with a yellow background.
doi:10.1371/journal.pone.0072122.g001
Papillons and 93 Phalènes. In addition, the mutation was studied
in 121 dogs from other breeds affected with PRA or retinal
degeneration, representing 44 breeds (Table S3) and 334 healthy
control dogs from 10 breeds, including Australian Shepherd
(n = 20), Border Collie (n = 92), Chihuahua (n = 19), Finnish
Lapphund (n = 93), Japanese Chin (n = 14), Shetland Sheepdog
(n = 20), Silk Terrier (n = 20), Toy Poodle (n = 20), West Highland
White Terrier (n = 20) and Wire Haired Dachshund (n = 16).
The PCR reactions were carried in 12 ml reactions consisting of
0.6 U Biotools polymerase (Biotools, Madrid, Spain), 200 mM
dNTPs (Thermo Scientific), 1.5 mM MgCl2 (Biotools), 1 x PCR
buffer (Biotools), 0.83ml of forward and reverse primers (Sigma
Aldrich) and 10 ng template genomic DNA or mRNA. The
reaction mixtures were subjected to a thermal cycling program of
95uC for 10 min, followed by 35 cycles of 95uC for 30 s, 30 s at the
annealing temperature 60uC, and 72uC for 60 s and final
elongation stage of 72uC for 10 min. The PCR products were
purified using ExoSap (Thermo Scientific) according to manufacturer’s instructions and sequenced using the Sanger sequencing.
Sequence analysis was performed using Sequencher software
(Gene Codes, Ann Arnbor, MI, USA).
The CNGB1 mRNA sequence (XM_848817.1) was amplified
using Biotools (Biotools) polymerase and PCR protocol described
above with annealing temperature of 62uC. The primers used for
amplification are listed in supplementary data (Table S1).
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Results
We established a cohort of PRA affected and unaffected
Papillons and Phalènes to map the disease locus and to identify the
genetic cause. Clinical signs reported from the affected dogs were
consistent with PRA, including tapetal hyper-reflectivity and
vascular attenuation. The average age of the PRA diagnosis was
4.3 years. The pedigree established around the affected dogs
suggested a recessive mode of inheritance (Fig. 1).
We first studied the retinitis pigmentosa GTPase regulator interacting
protein 1 (RPGRIP1), originally associated with PRA in the
Miniature Longhaired Dachshunds as a candidate gene in our
study cohort using the described PCR conditions for the mutation
[11]. We found a high carrier frequency in the breed 17.1 % (25/
146), but only one affected dog was homozygous for RPGRIP1
mutation. This leaves the role of RPGRIP1 unclear and we,
therefore, initiated a genome-wide association and linkage analyses
to map the PRA locus in a cohort of 6 PRA affected and 14
unaffected dogs. Control dogs were ophthalmologically normal
when examined at greater than seven years of age. Statistical
analysis by PLINK [20] identified the most highly associated SNP,
BICF2P309315 in CFA2 at 61428709 bp (praw = 4.761026,
pgenome = 0.1) (build 2.1 of the canine genome reference sequence)
(Fig. 2A). The association was supported by six closely positioned
markers. Another tentative region with two separate SNPs
positioned from 70352612 bp to 77628107 bp was found in
CFA1 with the best associated SNP, BICF2P839236
(praw = 7.061026, pgenome = 0.2). Re-analysis of the data by
adjusting for the strongest SNP (BICF2P309315) on CFA2 did
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after 4 years of age and none of them have been diagnosed with
PRA. These results indicate a highly significant association
between the mutation and disease (PFisher = 1.461028) when
comparing genotyped homozygous affected dogs (n = 6) and dogs
free of PRA using Fisher’s exact test. The healthy dogs included 4
dogs homozygous for the mutation, 25 heterozygous and 116 dog
homozygous for the reference allele.
To gain further support for the causality of the CNGB1
mutation, we screened additional 334 healthy dogs from 10 other
breeds but the mutation was not found. In addition, the mutation
was studied in 121 dogs from 44 other breeds affected with PRA or
retinal degeneration (Table S3), but none of these affected dogs
had the CNGB1 mutation. Collectively, these results indicate that
the CNGB1 mutation is causal and breed-specific in Papillons and
Phalènes.
The entire CNGB1 mRNA was successfully amplified from a
retinal sample from a PRA-free Australian Cattle Dog [Genbank
Accession KF551910], indicating that the canine CNGB1 is also
transcribed in the retina. We did not have access to retinal samples
from the affected Phalenes or Papillons.
not support the association on CFA1 (BICF2P839236, p = 0.9)
confirming the association on CFA2. In addition, genotyping of
the best associated SNP on CFA1 in a larger cohort of 6 cases and
168 controls (p = 6.9761025) did not improve the original
tentative GWAS association (p = 1026) and strongly suggest that
CFA1 does not play a role in the disease etiology. No significant
population structure was identified by genome wide IBS clustering
(genomic inflation factor = 1).
The association results were confirmed by a joint family-based
linkage and association analysis using a Pseudomarker program.
The joint analysis identified the most highly linked region covering
nine SNPs at the same locus on CFA2 between 61198715 bp to
64929333 bp, (linkage p = 1.061026, association p = 8.761026
and joint analysis p = 1.961027) (Fig. 2B). A weaker association
was identified also in CFA1 (linkage p = 1.061026, association
p = 2.061026 and joint analysis p = 2.061026) (data not shown).
Assessment of genotypes at the associated CFA2 locus revealed
a shared 1.9 Mb homozygous haplotype block in all of the affected
dogs (Fig. 2C), with recessively segregating markers between
61420765-61645929 bp. The associated region contains 37 genes,
including a known human RP gene, CNGB1.
Based on the exome sequencing data, on average 97 % of the
generated raw reads were aligned to CanFam2.1 reference
sequence per individual. Of these reads 76 % on average, were
aligned to the target region. Each individual had a mean depth of
95 reads on average in the targeted region and 98.9 % of the
targeted region was covered with at least one read and about 74 %
of the target regions were covered with at least 20 reads (Table S2).
The variant calling algorithms detected altogether 129590 single
nucleotide variants (SNVs) and 25905 indels across the three
individuals. After analyzing the trio according to a recessive mode
of inheritance (an affected proband and healthy parent dogs), 204
SNVs and 9 indels were identified. Of the coding variants, two
were located in the associated region on CFA2: a 1-bp deletion
followed by a 6-bp insertion in the CNGB1 gene (c.2685delA2687_2688insTAGCTA) (Fig. 2D) and a non-synonymous
c.1128G.A, p.376M.I mutation in the OGFOD1 gene. The
indel variant in CNGB1 causes a frameshift and a premature stopcodon p.Tyr889Serfs*5 in an evolutionary conserved region (Fig.
3A and B), while the missense mutation in OGFOD1 was predicted
to be benign based on the bioinformatics prediction using
Polyphen2 [29] and SIFT-programs [30]. We further analyzed
the exome data for the region on CFA1, but did not have any casespecific variants, supporting the CFA2 as the causative locus and
CNGB1 mutation as causative for the PRA in the breeds.
To gather further evidence for the CNGB1 mutation, we
genotyped all six PRA affected dogs and all were homozygous
indicating a full segregation with the disease. We also genotyped
four obligatory carriers from the pedigree (Fig. 1) and they were all
mutation carriers. We then genotyped additional 145 randomly
selected unaffected Papillons and Phalènes and found four
additional dogs homozygous for the mutation and a 17.2% carrier
frequency (25/145) was indicated. Of the four homozygous dogs
found in the population screening, one was reported to have vision
problems in the dim light, which may indicate that the dog is
indeed affected with PRA. However, the dog has not had
ophthalmologic examination. This dog’s full sibling was also
homozygous but neither has been ophthalmologically examined.
Two other homozygous dogs had been ophthalmologically
examined at very young age (1-2 years), well before the average
age of onset of the PRA in the breed and it is likely that the clinical
signs are not present at this age. The clinical follow up of these
dogs would be important in coming years. We found also 25
carriers of which 52 % have been ophthalmologically examined
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Discussion
We have performed a series of genetic analyses, including
GWAS, linkage and exome sequencing to identify the mutation for
PRA in Papillon and Phalène breeds. Although we managed to
establish only a small cohort and pedigree of cases and controls, we
mapped a 1.9 Mb locus at CFA2, and confirmed it by both
association and linkage approaches according to our pedigree
analyses with the expected recessive mode of inheritance. The
associated region at CFA2 contains 37 genes, including a known
human retinitis pigmentosa gene, CNGB1. It is clinically and
functionally highly relevant candidate gene for PRA in the
Papillon and Phalène breeds. Importantly, our exome data,
covering all the coding regions of the associated locus, revealed
CNGB1 to be the only gene with a disease segregating pathogenic
mutation: a complex indel variant was found in exon 25 resulting
in a premature stop codon and predicting nonsense mediated
decay (NMD) of the CNGB1 mRNA. NMD eliminates the
production of mRNA that contains premature translation
termination codon and code for nonfunctional polypeptide. In
mammalian cells nonsense codons are recognized after splicing
and mRNA is programmed to NMD, if there is at least one
splicing generated exon-exon junction .50-55 nucleotides downstream of the premature termination codon [31,32]. As the
identified mutation in the CNGB1 is on exon 25 and the
c.2685delA2687_2688insTAGCTA variation is 127 nucleotides
from the nearest exon-exon junction downstream, it is hypothesized that the mRNA is subjected to NMD and no protein is
translated.
The CNGB1 gene encodes for a rod photoreceptor cyclic
guanosine monophosphate-gated (cGMP) channel b-subunit and
is a part of the rod photoreceptor cGMP-gated cation channel
complex [33]. This complex consists a and b-subunits and form
tetrameric cyclic nucleotide gated (CNG) channels [34], [35]. The
visual transduction cascade is mediated by this non-selective cation
channel, which is directly gated by cGMP [36]. The channel
controls for the flow of sodium and calcium ions through the
plasma membrane in response to light induced changes in cGMP
level [33]. The b-subunit contains a channel-like structure, but
does not form functional subunit without the co-expression of the
a-subunit [34], [33]. The a-subunit contains six membrane
segments, a voltage sensor region, a pore region and a cGMPbinding motif [37].
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Figure 2. Genome wide association and linkage analyses. A) A Manhattan plot of genome-wide case-control association analysis performed
using 6 cases and 14 controls indicate the most highly associated region in CFA2. B) The PRA associated region on chromosome 2 spans from
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61.4 Mb to 63.3 Mb based on association, linkage and joint analyses. C) Genotypes at the PRA associated region on CFA2. All cases share a 1.9 Mb
homozygous block, and within this block SNPs BICF2S23238410, BICF2P309315 and BICF2P75954 show complete recessive segregation with the
disease. D) Chromatograms of the c.2685delA (arrow), c.2687_2688insTAGCTA (shadowed) mutations in CNGB1 gene in an affected (2) and normal (1)
dog. The CNGB1 gene is located between the two segregating SNPs (BICF2P309315, BICF2P75954).
doi:10.1371/journal.pone.0072122.g002
lack functional data due to limited access to retinal tissues from the
affected dogs, it is likely that the mutation abrogates CNGB1
functions and causes retinal degeneration in Papillons and
Phalènes. These breeds provide a model to study the genotypephenotype correlations related to the possible alternative transcripts and their potential tissue-specific functions.
The defective CNGB1 has been implicated as a cause of retinal
degeneration in human and mouse. Bareil et al. identified a
p.G993V missense mutation is a consanguineous French family
affected by a night blindness and progressive loss of peripheral
vision and ERG recorded rod responses [42]. In another study, a
splice site point mutation (c.3444+1G.A) was found in a patient
with night blindness, retinal pigment lesion and non-recordable
dark adapted flash ERG [43]. In addition, a missense mutation
(c.2957ART; p.N986I) was found in a patient with typical RP
with attenuation of the retinal vessels, retinal pigment atrophy and
optic disc pallor [44].
In the CNGB12/2 mice, the CNGB1 gene was found to be
important for the formation and outer segment targeting of the rod
The human CNGB1 comprises of 33 exons and several different
transcripts have been found in different tissues [38]. In the retina,
at least seven different transcripts are expressed [38]. Rod cells
contain a long isoform of the CNGB1 (CNGB1a) [39]. The
shorter isoform (CNGB1b) is expressed in the olfactory CNG
channels [40] as well as sperm cells and other tissues [41]. The Nterminal part of CNGB1 between exons 1-16 is a glutamic acidrich and it is also expressed as a separate protein (GARP) [38].
GARP is highly enriched in rod photoreceptors [38]. The Cterminal part is homologous to the a-subunit and forms the
channel-like portion. Exons 21-26 are homologous to the
transmembrane and pore domains present in the a-subunit, exons
29-31 encodes for the cyclic nucleotide binding domain (CNBD),
and exons 19 and 32 codes for domain that are important for the
calcium calmodulin/regulation of cGMP affinity [38].
The canine CNGB1 is similar to human and contains 32
predicted exons based on mRNA (XM_848817.1) sequence
alignment. The frameshift mutation in exon 25 predicts that the
CNGB1 mRNA is destroyed by NMD. Therefore, although we
Figure 3. CNGB1 protein alignments. A) CNGB1 amino acid alignment of the normal and affected dogs. The p.Tyr889Serfs*5 mutation in the
affected dog results in a loss of a significant part of the C-terminus of the protein and probable NMD of the CNGB1 mRNA B) CNGB1 sequence
alignment between different vertebrates. The mutation is located in a highly conserved region across species. The arrows mark the first mutated
amino acid caused by the frameshift and the premature stop codon.
doi:10.1371/journal.pone.0072122.g003
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CNG channels. The CNGB1-deficient mice developed retinal
degeneration that resembles human RP, progressing from the rod
degeneration to a complete loss of rod function and secondary
degeneration of the cones [45]. However, the disease in CNGB12/
2
mice progresses more slowly when compared to other RP mouse
models and the exact events that lead to rod cell death are unclear
[45]. In CNGB12/2 mice, 80290% loss of the rod responses is
not detected until at the age 1 year [46]. In human patients, night
blindness is usually reported at school age but RP is commonly
diagnosed around 30 years of age [42,43] The PRA in Papillon
and Phaléne breeds is diagnosed at the middle age dogs and
progresses similar to human and mice.
In summary, we have discovered a putative genetic cause of
PRA in the Phalène and Papillon breeds by identifying a
frameshift mutation in CNGB1. As a further confirmation, the
same mutation was also recently reported in an independent study
by Petersen-Jones et al. (2013) at the Association for Research in
Vision and Ophthalmology (ARVO) conference (unpublished
data). Our study enables the development of a genetic test to
eradicate the disease from the breeds. The breeds have a high
carrier frequency (17.2 %) and careful breeding plans by mating
carriers to normal should be considered to maintain genetic
diversity. Importantly, our study establishes a large functional
knockout animal model for potential therapeutic trials such as
gene therapy, and provides a relevant model for further studies of
the CNGB1 biology in the retina and other tissues.
Supporting Information
Table S1 Primers used for PCR amplification and sequencing of
canine CNGB1 mRNA.
(XLSX)
Summary of the exome sequencing data analysis of
each sequenced individual dog.
(XLSX)
Table S2
Table S3 Dogs affected with PRA (44 breeds, total 121) were
sequenced for the CNGB1 mutation.
(XLSX)
Acknowledgments
We would like to thank all owners that donated samples to our DNA-bank
from their dogs. The Finnish Papillon and Phalène Club is thanked for
their co-operation. Minna Virta, Ranja Eklund, Sini Karjalainen, Heta
Haikonen and Elina Vuorenmaa are thanked for their technical support
and help in sample recruiting and pedigree analyses.
Author Contributions
Conceived and designed the experiments: SA. Performed the experiments:
SA MA. Analyzed the data: SA MA. Contributed reagents/materials/
analysis tools: SA MA. Wrote the paper: SA HL.
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